Electrosensitive printing is a well-known form of nonimpact printing technology. A sheet or web of paper having a metallized surface, generally a thin layer of aluminum, is subjected to electric current from a printing element having one or more electrodes that lightly contact the paper. The electrodes may be wires or a metal element such as of tungsten etched to define individual electrodes. Within an extremely short time, depending upon the voltage applied to the printing element and the manner in which it is applied, the metal layer is vaporized in controlled patterns to expose an underlying dark layer and thereby form the desired images.
Electrosensitive printing offers advantages of mechanical simplicity, quiet high speed operation and low equipment and maintenance costs as compared to impact printing and other forms of nonimpact printing. For example, electrostatic nonimpact printers are considerably more expensive than electrosensitive printers, and thermal nonimpact printers are substantially slower than electrosensitive printers because of the time required to form the images. The technology originally was employed with chart recorders to form a line image with a stylus to which electric current was applied. Printing of alphanumeric characters and simple graphic images evolved through the development of printheads capable of printing dot matrixes. More recent electrosensitive printing improvements have made possible the formation of complex graphic and facsimile images with electrosensitive papers.
The relative shiny or metallic appearance of early electrosensitive papers was considered a disadvantage by a number of users. This problem was resolved by several paper manufacturers by the expected technique of incorporating flatting agents in the dark layer underneath the metal layer to thereby reduce the glossiness of the finished paper, such as was commonly done to reduce gloss of other types of coatings. Some examples of this technique are also disclosed in the patent literature, see for example U.S. Pat. Nos. 3,786,518 and 3,861,952. Another proposed approach was to utilize a paper substrate having a rough surface texture, e.g. U.S. Pat. No. 3,758,336.
A typical modern use of electrosensitive papers is in conjunction with a multielectrode printhead controlled by a microprocessor as a printer associated with a computer terminal. The printhead consists of several electrodes tightly packed in close array; it is capable of printing in a dot matrix by microprocessor control of current to individual electrodes in the printhead. Printheads of this type are capable of high resolution image-formation, and the size of a printed mark can be adjusted by controlling the duration of the pulse of current or the applied voltage. However, additional burdens have been placed on electrosensitive papers because of the necessity to provide a paper that is capable of controlled image size in order to enable the formation of high resolution images. Another added requirement is the need to provide for printhead cleaning action so that the printhead will not become fouled by debris from the paper collecting within the interstices between individual wires of the printhead. Furthermore, electrosensitive papers need to have a high degree of thermal stability in order to permit the greater image density associated with some printheads of this type.
It has been found during the course of the research and development work resulting in the present invention that electrosensitive papers having surface roughness designed to reduce metallized gloss, such as described in the aforementioned patents, often interfere with the ability to completely remove the metal layer from the image areas, fail to precisely control image size and shape to the extent necessary for high resolution printing, and do not afford suitable cleaning action and/or thermal stability to prevent fouling of the printhead. One of the principal objects of this invention is to provide an electrosensitive recording paper that does not have the disadvantages of the prior papers.
Another principal object of this invention is to provide a new electrosensitive paper capable of high density, high resolution images. Yet another principal object is to provide an electrosensitive paper having controlled heterogeneous surface topology which is developed by providing a substrate having a very smooth surface and developing the desired surface topology through the structure of a colored coating on the substrate underneath the metallized layer. These and other objects will become apparent in the detailed description which is presented below.